1. Technical Field
The present invention generally relates to lubricating oil compositions for reducing wear in engines.
2. Description of the Related Art
Phosphorus, particularly the phosphorus delivered by zinc dialkyldithiophosphate (ZDDP), has been the predominant antiwear agent in fully formulated lubricants for the past 50 years. Studies have suggested that phosphorus may poison catalytic converters used in gasoline-fueled engines to reduce exhaust emissions of unburned hydrocarbons and oxides of nitrogen, see, e.g., Spearot et al., “Engine Oil Phosphorus Effects on Catalytic Converter Performance in Federal Durability and High Speed Vehicle Tests,” SAE Technical Paper 770637 (1977); Caracciolo et al., “Engine Oil Additive Effects on the Deterioration of a Stoichiometric Emissions Control (C-4) System,” SAE Technical Paper 790941 (1979); and Ueda et al., “Engine Oil Additive Effects on Deactivation of Monolithic Three-Way Catalysts and Oxygen Sensors,” SAE Technical Paper 940746 (1994). As the environmental regulations governing tailpipe emissions have tightened, the allowable concentration of phosphorus in engine oils has been significantly reduced with further reductions in the phosphorus content of the engine oils being likely in the next category, i.e., GF-5 to perhaps 0.05 wt. %.
Many partial solutions exist, where either Zn, P or S have been partially or totally eliminated. In one approach, Zhang et al., “Tribofilms Generated From ZDDP and ashless dialkyldithiophosphate (DDP) on Steel Surfaces, Part 1, Growth, Wear, and Morphological Aspects,” Tribology Letters, Vol. 19, 3, pp 211-220 (2005) studied the growth and morphology of tribofilms generated from ZDDP and a DDP over a wide range of rubbing times (10 seconds to 10 hours) and concentrations (0.1 to 5 wt. % ZDDP), using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption near edge structure (XANES) spectroscopy at the O, P, and S K-edges and the P, S, and Fe L-edges. The major components of all films generated using a Cameron-Plint tester on 52100 steel are Zn and Fe phosphates and polyphosphates. The average thickness of these phosphate films has been measured using P K-edge XANES and XPS profiling. For ZDDP, a very significant phosphate film (about 100 Å thick) forms after 10 seconds, while film development for DDP is substantially slower. However, for both additives, the average film thickness increases to 600 to 800 Å after 30 minutes of rubbing, before leveling off or decreasing.
The antiwear properties of pure ZDDP and in combination with DDP at different rubbing times and concentrations have also been examined. It was found that under all conditions, the performance of ZDDP as an antiwear agent is superior to that of DDP. However, DDP has no adverse effect on the performance of ZDDP when the two are mixed, suggesting that DDP can be used with ZDDP, thereby reducing the amount of total ash.
U.S. Pat. No. 5,405,545 discloses a lubricant additive having antiwear and antioxidant properties and is the reaction product of a thiodicarboxylic acid and an ether amine which is post-reacted with an aliphatic alcohol, an aliphatic amine, and/or a trialkylphosphite.
U.S. Pat. No. 5,674,820 (“the '820 patent”) discloses a composition comprising: (A) a compound represented by the formula:
wherein R1, R2, R3, and R4 are independently hydrocarbyl groups, and X1 and X2 are independently O or S, and n is 0 to 3; and (B) an acylated nitrogen-containing compound having a substituent of at least 10 aliphatic carbon atoms. The '820 patent further discloses that the composition can contain a second phosphorus compound other than (A), with the second phosphorus compound being a phosphorus acid, phosphorus acid ester, phosphorus acid salt, or derivative thereof.
The above references largely describe P- or S-containing supplemental wear inhibitors. Unfortunately, the tightening of emission requirements requires wear inhibitors with relatively no P, S, and/or Zn content. Trialkylsilanes have been disclosed to add thermal stability to lubricants in U.S. Pat. No. 4,572,791 and phenyltrialkylsilanes were disclosed for oxidation improvement in U.S. Pat. No. 5,120,485. Trifunctional hydrolysable silanes have found some applications in fuels and lubricant compositions, e.g., U.S. Pat. No. 4,541,838 discloses additive mixtures of an organic nitrate ignition accelerator and a trialkoxysilane for use in fuel compositions. U.S. Pat. No. 6,887,835 discloses bis-(trialkoxysilyl)alkyl polysulfides as well as other linking groups including polysiloxanes. The bis and polymeric silane compounds showed a reduction in the Falex 4-ball wear scar using the ASTM D 4172 test.
U.S. Patent Application Publication Nos. 20080058231 (“the '231 application”) and 20080058232 (“the '232 application”) disclose a lubricating oil composition containing (a) a major amount of an oil of lubricating viscosity; and (b) a tetra functional hydrolyzable silane. Each of the examples in the '231 and '232 applications further disclose that the lubricating oil compositions contain a zinc dihydrocarbyl dithiophosphate.
Russian Patent No. SU-245955 (Jun. 11, 1969) discloses lubricant additives which improve the antifriction and anticorrosion characteristics of lubricating oils when used in amounts of 2 to 35 wt. %, and preferably 5 wt. % are trialkoxyorganosilanes of the general formula (AlkO)3SiRR′ (where AlkO is an alkoxy group, R is alkyl, aryl or alkenyl group, and R′ is a functional group such as such as NH2, CO2H, COH, OH, or CN).
Great Britain Patent No. 1 441 335 discloses lubricant compositions to improve antifatigue containing about 0.01 to 5% weight of a condensation polymer derived from a trialkoxysilanes of the formula R—Si(OR1)3 where R is a C1-12 alkyl or C2-24 alkoxyalkyl, and R1 is a C1-12 alkyl or C2-12 alkoxyalkyl, where alkoxyalkyl means an ether group represented by —Cn—O—Cm— wherein the sum of n plus m is 2 to 24 in the case of R and 2 to 12 in the case of R1.
Japanese Patent Publication No. 8-337788 (Dec. 24, 1996) (“the '788 publication”) discloses additives consisting of silane compounds, e.g., (a) R1Si(OR)3, (b) (R1)2Si(OR)2, and (c) (R1)3SiOR wherein R is H, C1-18 alkyl, C2-18 alkenyl, C6-18 aryl; and R1 is C6-50 alkenyl optionally containing a N, O, and/or S atom or substituted with hydroxyl, carbonyl, alkoxycarbonyl, alkenoxycarbonyl or aryloxycarbonyl, or a C6-50 aryl.
Accordingly, as demand for further decrease of the phosphorus content and a limit on the sulfur content of lubricating oils is very high, this reduction cannot be satisfied by the present measures in practice and still meet the severe antiwear and oxidation-corrosion inhibiting properties required of today's engine oils. Thus, it would be desirable to develop lubricating oil compositions, and additives and additive packages therefor having relatively low levels of phosphorus and sulfur, and no zinc but which still provide the needed wear and oxidation protection now provided by lubricating oils containing a zinc dialkyl dithiophosphate.